As recycling of aluminum increases, melting losses still are as much as 10% of the dross weight, representing an enormous loss of material and energy. Loss of valuable metals as a result of the formation of suspensions of both fine and relatively large droplets of molten metal in the molten (very thick or viscous) salt solution is the major concern in the secondary aluminum recycling industry. The most common method conventionally used to minimize the oxidation losses of metal such as aluminum is the use of a protective molten salt cover, called flux, in the charging well of the reverberatory furnace generally used for melting scrap metal. Such a molten salt also is added in a rotary furnace generally used for recycling skims and drosses, and these may contain as much as 85% metallic aluminum. The molten salt flux serves the multiple purposes of protecting aluminum from further oxidation and/or combustion, stripping the protective oxide film from the molten metal so the droplets can more easily coalesce, and holding the stripped solid oxide particles in suspension so that a clean metal can be recovered.
It is common in such secondary aluminum remelting to use a mixture of equimolar sodium and potassium chlorides as the salt flux because of the low cost and the low eutectic temperature in this system. A small amount of fluorides, such as KF, NaF, cryolite, etc., is frequently added to the binary mixture of the chlorides. Although aluminum oxide, if formed, may be dissolved by fluorides, such as cryolite, the presence of sodium chloride in molten salts lowers the solubility of the aluminum oxide to insignificant levels. Thus, the action of the salt flux in assisting coalescence of metal droplets results from the stripping of the oxide films rather than the actual dissolution of the oxide. It is believed that the stripping is due to the interfacial stress created by interfacial tension gradient. The interfacial tension depends not only on the interfacial potential between the molten metal and molten salt, but also on the surface charge in the metal droplet surface. This phenomenon is known as electrocapillarity, but has not been developed in metal-salt systems because of experimental difficulties.
The salt forms a cover which thickens over time. This salt cover, or dross, prevents oxidation of the molten metal beneath and accumulates the stripped oxides, such as magnesium oxide and aluminum oxide. When the dross becomes very thick, it is usually removed but it contains a significant amount of entrapped molten aluminum, thereby reducing the aluminum yield. Conventionally, large rollers are used to squeeze the removed dross to thereby recover about half of the trapped metal, however, the dross still contains 10% metal which is not recovered and is lost.
Stirring of the molten metal/molten salt mixture to improve the yield of metal is not practical on a commercial scale because of the size of the vessels. Therefore, it would be desirable to have a method for mixing the materials and thereby allow the small molten metal droplets to coalesce into larger particle sizes to increase the recoverable yield.